Computing devices such as smartphones, tablets, e-readers, smart watches, virtual keyboards, machine panels and the like, can include a touch system in communication with a control unit configured to recognize the touch signal from the touch system and the control unit can take contextual action based on its programming. Touch systems can sense a user touch that can be from one or more fingers or styluses. A user touch becomes a touch input when the touch is recognized by a computing device and may be used to take action. An action received by the user is a touch output. Touch systems such as touch screens, touch pads and touch controls and the like are well known in the art for manipulating computing devices such as portable and desktop devices, gaming systems, and industrial machines. Touch screens can have a touch sensing panel which is transparent, include a touch sensing system arranged over a display and can sense a user touch. Displayed information can not only be read by the user but the user may even interact with it by touching the touch screen on specific locations to initiate an action. Touch pads can have a touch sensing panel that may or may not be transparent and can sense a user touch. User touch over the touch pad area may become a location specific action on an associated but separate display. Touch controls may have a touch sensing panel with discrete arrangements of touch buttons or touch input keys or sliders. Touch controls maintain the aesthetics and integrity of the panel's material and design and can still provide the necessary buttons for user input. Touch systems includes a touch sensing panel, may include a protective cover layer, support frames and substrates, and may or may not include one or more displays, other buttons and the like.
Touch sensing panels can be formed by projected capacitive sensing, commonly referred to as capacitive touch. Here an array of conductive metal lines is orthogonally arranged and separated by an insulating layer creating a capacitive pixel at the cross points. An electrically activated capacitive array can then detect a user touch and its location, specifically the user touch interferes and attenuates the array's electrical signal pattern over a few pixels. Projected capacitive sensing requires the user to touch by finger or special conductive stylus because the signal attenuating interference needed to recognize a user touch is based on an electrical signal being diverted by creating a temporary conducting path to ground through the user. The user touch is recognized by the computing device; specifically the touch sensing panel communicates the user touch to the control unit where the programming determines the necessary context based action. Capacitive touch is a two-dimensional touch system that recognizes the surface location of user touch; specifically the x-y coordinates of touch. Touch sensing panels may be capable of more than a single user input. They may be capable of multi-touch and gesture touch. In single user touch only one touch point on the touch sensing panel is recognized at any given instance as compared to multi-touch where more than one touch may be simultaneously be recognized by the touch system. Multi-touch may be used to develop touch patterns or gestures, for example two fingers being moved apart may be a zoom-in gesture or two fingers being brought together may be a zoom-out gesture.
Force sensing touch systems that are capable of force sensing add new capabilities to user input. Not only can the touch system know the touch location, it can also become aware of how hard the user is pushing. The additional degree of freedom can create more innovative interactions with touch systems and make them more intuitive to the user. One handed operations are more easily implemented, for example moving the web page up or down can be dynamically adjusted based on the applied force or music volume can be turned up or down quickly based on applied force. User applied force leads to mechanical changes or strain detected by the force sensing touch system and converted into an electrical signal for the computer system to recognize and act upon. Force sensing touch systems require integrating force sensors with touch sensing panels and incorporating signals from force sensors into the related computing device organization and operations. Special care must be taken to account for space and mechanical behavior of the physical design of the product to ensure that the user applied force and its associated strain is optimally detected by the force sensors. True force sensing touch systems may not be formed by using projected capacitive touch. Force sensing touch systems may use discrete mechanical springs, force sensing resistors (FSR), force sensing capacitors (FSC), strain gauges (SG), or piezoresistive sensors. Discrete mechanical springs and discrete strain gauges are generally large in size on the order of few millimeters or more, require tens of microns of deflection for activating the force sensors and have low sensitivity. Force sensing resistors and force sensing capacitors are very sensitive to preloading mechanical stresses introduced during the product assembly in manufacturing and can lead to yield losses, sensor variability, and reduced dynamic operating range. Semiconductor strain gauges, cantilevers and membranes have been developed as force sensors directly integrated with the LCD display during semiconductor manufacturing, but these require special designs and can be relatively expensive. Piezoresistive sensors have improved tolerances to manufacturing stresses and are capable of useful functioning when incorporated as edge or corner sensors within the touch system but this may require redesign from material selections to assembly process to ensure optimal integration of force sensors. For larger touch systems, even the edge/corner piezoresistive sensors face challenges with spatial resolution and mechanical robustness. Hence, there is a need for sensitive and robust force sensing touch system that is low cost and capable of modular integration into existing designs.